The invention is directed to a method for determining the moment of inertia of at least an element of a drive train of a vehicle, particularly of an internal combustion engine on a testing stand, whereby the observed torque (M.sub.M) is corrected according to the equation: M=M.sub.M +M.sub.V by the loss moment (M.sub.V) effective at the time, and the moment of inertia is determined from the corrected torque M and the angular acceleration n according to the equation I=M.cndot.(30/.pi.n) or equivalent quantities.
In this context, the exact value of the moment of inertia is an important but often only inadequately known quantity. The determination thereof is of relatively great significance for research and development, and also, for example, for error diagnosis and maintenance, especially in conjunction with internal combustion engines for vehicles. In various other contexts it is also of interest to know the moment of inertia of a gearing alone or of combined elements of a drive train. Obtaining the moment of inertia can also be important for research and development.
For example, knowledge of the moment of inertia is of particular significance for the design of the connecting shaft between an internal combustion engine to be tested and the testing stand and for the parameterization of the testing stand or control circuits. In this context, incorrectly assumed moments of inertia can lead to poor control and regulating behavior of the testing stand or can even lead to operating malfunctions due to breakage of the shaft as a result of dynamic overload. Knowledge of the moments of inertia is also important, for example, considering large ship engines since, in particular, the elastic bearing of the motor must be designed for the reaction moments of the motor that occur during the start-up procedure.
In this context, for example, EP-A-434 665 discloses that the unknown moment of inertia be determined as: I.sub.o =I.sub.1 .cndot.w.sub.1 /(w.sub.o -w.sub.1) from a measurement of the angular acceleration with and without known supplemental moment of inertia. In order to manage without a known supplemental moment of inertia, Newton's known law (force=mass.times.acceleration) for the motion of revolution is utilized as the determining context for the calculation of the moment of inertia. Accordingly, the moment of inertia I can be determined as: I=M.cndot.(30/.pi.n) as quotient of the measurable quantities of torque (M) [Nm] and angular acceleration (n) [min.sup.-1 /sec], whereby averaging can be carried out as needed over a number of rotational cycles for eliminating the superimposed torsional oscillations and rotational inequalities. This simple relationship, however, is only valid for the absolute loss-free case. In order to take the losses determined by various causes into consideration, the measured torque (M.sub.M) or, respectively, the indicated torque (M.sub.M) or, respectively, the torque (M.sub.M) that can be identified in some way or other must be corrected by the loss moment (M.sub.V) effective at the time (the frictional moment (M.sub.R) in the simplest case) according to: M=M.sub.M +M.sub.V where M is the corrected torque.
According to the prior art known in this context, the frictional moment or the frictional power is determined at, for example, the motor testing stand, for example in drag operation with respectively constant speed or in coasting with different centrifugal masses such as balance weights. What is referred to as friction characteristic in this context is the dependency of the frictional moment identified in this way on the rpm. In these known methods, the frictional moment is in fact generally assumed to be rpm-dependent but, however, essentially constant for the duration of the measurement. Since, however, it is dependent on many different influencing variables, for example on the oil temperature that generally changes during the measurement, such methods and their application to the determination of the moment of inertia involve great errors.
EP-A-199 431, for example, is relevant in this context. According to this document, the momentary frictional moment can be determined, for example, from the indicated cylinder pressure and the rpm curve, whereby the oscillating and the rotating moment of inertia are assumed to be known. A reversal of this method for determining the moment of inertia itself, however, is not practical. In the case of an electric motor running practically friction-free, further, it is known from, for example, EP-A-476 588 to determine the moment of inertia I from an acceleration phase (positive or negative) according to: I=.intg. M dt/.DELTA.w by integration of the measured torque (M) and division by the speed difference (.DELTA.w).